Unobstructed construction has horizontal structural members that are not solid and have openings at least __________ percent of their cross-sectional area.

In the context of unobstructed construction, having horizontal structural members that possess openings is crucial for allowing fire suppression systems, such as sprinklers, to operate effectively. The requirement for these openings is based on the concept that a certain percentage of the cross-sectional area must be free of solid material, which facilitates the distribution of heat and smoke and aids in the activation of sprinklers.

The correct answer indicates that these openings must account for at least 70 percent of the cross-sectional area of the structural members. This threshold ensures that the area is sufficiently unobstructed to allow for proper fire dynamics and to enable sprinklers to distribute water effectively throughout the space below any horizontal members, which is essential for maintaining safety in building design and fire protection systems. By adhering to this requirement, building designs promote better ventilation and fire control, aligning with fire safety standards and regulations.

Options indicating a lower percentage, such as 40, 50, or 80 percent, do not meet the criteria needed to ensure adequate performance of sprinkler systems in an unobstructed construction, thus emphasizing the importance of the 70 percent threshold.